Prokaryotes and eukaryotes
Cells can be either prokaryotic or eukaryotic. Eukaryotic cells are complex and include all animal and plant cells. Prokaryotic cells are smaller and simpler, e.g. bacteria.
Eukaryotes are organisms that are made up of eukaryotic cells. A prokaryote is a prokaryotic cell (it’s a single-celled organism). Both eukaryotic and prokaryotic cells contain various cell parts called subcellular structures.
Most animal cells have the following subcellular structures — make sure you know them all. The parts are labelled in the diagram below.
- Nucleus — contains genetic material that controls the activities of the cell.
- Cytoplasm - a gel-like substance where most of the chemical reactions happen. It contains enzymes that control these chemical reactions.
- Cell membrane - holds the cell together and controls what goes in and out.
- Mitochondria - these are where most of the reactions for aerobic respiration take place. Respiration transfers energy that the cell needs to work.
- Ribosomes — these are where proteins are made in the cell.
Plant cells usually have all the bits that animal cells have, plus a few extra:
- Cell wall - a rigid structure made of cellulose. It supports and strengthens the cell. The cells of algae (e.g. seaweed) also have a rigid cell wall.
- Permanent vacuole — contains cell sap, a weak solution of sugar and salts.
- Chloroplasts — these are where photosynthesis occurs, which makes food for the plant. They contain a green substance called chlorophyll, which absorbs the light needed for photosynthesis.
The subcellular structures of a typical plant cell are shown in the diagram below.
Bacteria are prokaryotes. A bacterial cell has cytoplasm and a cell membrane surrounded by a cell wall. The cell doesn’t have a ‘true’ nucleus – instead, it has a single circular strand of DNA that floats freely in the cytoplasm – see diagram below. Bacterial cells may also contain one or more small rings of DNA called plasmids.
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